This invention relates to high-frequency electrical test instruments and, more particularly, to measuring instrumentation operable over a wide range of frequencies of interest. Specifically, the invention is directed to calibration of frequency-tuneable circuits used in broadband measuring instruments and provides, in one embodiment, a spectrum analyzer with built-in preselector calibration.
Variable tuning circuits, such as yttrium-iron-garnet (YIG) tuning elements, are commonly used in electronically tuneable band pass filters employed in microwave devices. In broadband microwave measuring instruments, such as receivers or spectrum analyzers, these tuneable band pass filters are often used in, or before, radio frequency (RF) input circuits as tuneable preselectors. Consequently, only a narrow band of frequencies is supplied to the instrument input at any one time. This preselection is particularly useful in a broadband heterodyne instrument, since preselection eliminates confusing spurious intermediate frequency (IF) signals due to the mixing of the local oscillator signal with input signals other than a signal having the frequency or frequencies of interest. Also, these tuneable band pass filters are used in sources or broadband swept frequency signal generators to provide a narrow band output signal.
To guarantee frequency and amplitude integrity in a receiver or spectrum analyzer, for example, both a first local oscillator (1st LO) and the YIG-tuned filter (YTF) need to track properly. The lst LO is driven such that its frequency, or harmonic of its frequency, is a fixed IF offset apart from the input frequency of interest. The YTF is driven so that it tracks the input frequency when the spectrum analyzer is tuned to frequencies covering a given frequency range, for example, 2.7 GHz and above. Because there is a fixed relationship between the positioning of the lst LO and the YTF center frequencies, previous analyzers offering preselection have typically used tune + span information driving a YIG oscillator in the lst LO as a source of information for tuning the YIG-tuned filter.
More particularly, a typical known circuit used to drive a YTF is shown in FIG. 1. A driver accepts the same tune + span signal as fed to the 1st LO, and processes it to include the IF offset, which can be either a positive or negative fixed frequency. The driver keeps track of which harmonic of the 1st LO is being used (whose multiplying effect is used to situate the YTF properly).
This known circuit allows a single tune + span signal for the 1st LO to provide information to drive the YTF as well. The price paid here is hardware in the driver to multiply tune + span for the given harmonic, and then extra hardware to accommodate the YTF's own sensitivity to the range allowed by the driver. This matching up is achieved either by an array of resistive potentiometers or digital-to-analog converters (DACS) to match gain and offsets on a per band basis to the given sensitivity of the YTF. This procedure is performed in a point-to-point calibration procedure at the factory and requires extensive manual interaction during the calibration process. In any event, however, the calibration is only approximate in that the calibration is typically determined by the peak response of the YTF over the given frequency range, which may not coincide with the center of the band due to the asymmetric frequency response characteristic of the YIG-tuned filter.